X-ray imaging is a common procedure, in medical imaging the energy range for the x-rays is typically 10 keV to 200 keV, in non-destructive testing or security screening the energy may be higher. In this range the x-rays reacts with matter mainly through Compton effect and Photoelectric effect. In the first case only a part of the energy of the x-ray photon is passed on to the electron and the x-ray continues with decreased energy after this scattering event. In the latter case all the energy is passed to the electron and the x-ray is completely absorbed.
The challenge for x-ray imaging detectors is to extract maximum information from the detected x-rays to provide input to an image of an object where the object is depicted in terms of density, composition and structure. It is still common to use film-screen as detector but mostly the detectors today provide a digital image.
The detector needs to convert the incident x-rays into electrons, this typically take place through Photo-effect or through Compton interaction and the resulting electron are usually creating secondary visible light until its energy is lost and this light is in turn detected by a photo-sensitive material. There are also detectors, less common, which are based on semiconductors such as amorphous Selenium or Silicon and in this case the electrons created by the x-ray is creating electric charge in terms of electrons and hole-pairs which are collected through an applied electric field with enough strength.
By far the most detectors operate in an integrating mode in the sense that they integrate the signal from a multitude of x-rays and this signal is only later digitized to retrieve a best guess for the number of incident x-rays in a pixel. The last years also so called photon counting detectors have emerged as a feasible alternative in some applications; currently those detectors are commercially available mainly in mammography. The photon counting detectors have an advantage since in principal the energy for each x-ray can be measured which yields additional information about the composition of the object; which can be used to increase the image quality and/or to decrease the radiation dose.
A very common configuration for the detector is as outlined in U.S. Pat. No. 7,471,765 “Cone Beam Computed Tomography With A Flat Panel Imager”. The detector incorporates a 512.times.512 array of a-Si:H photodiodes and thin-film transistors coupled to a scintillators and is operated in integration mode. In this case the application is to optimize radiation therapy but these detectors are also very common place in diagnostic imaging and other applications.
In U.S. Pat. No. 4,785,186 it is proposed an amorphous silicon detector for counting high energy particles including x-rays. U.S. Pat. No. 7,471,765 “Cone Beam Computed Tomography with A Flat Panel Imager”. This innovation was not exploited for x-ray imaging, maybe because of challenges with the amorphous silicon material in this application and also probably because of challenges with absorption efficiency.
Since in Photon Counting mode the signal from an individual x-ray is quite weak you need to maximize it by optimizing the conversion efficiency from x-ray energy into collected electric charge for each event. This means the use of Crystalline materials in detector is normally advisable. The advantages and the pit-falls with photon counting is described in Börje Norlin, “Characterisation and application of photon counting X-ray detector systems” Mid Sweden University Doctoral Thesis 26, ISSN 1652-893X, ISBN 978-91-85317-55-4 Electronics Design Division, in the Department of Information Technology and Media Mid Sweden University, SE-851 70 Sundsvall, Sweden and also in Mats Lundqvist Mats Lundqvist: Silicon Strip Detectors for Scanned Multi-Slit X-Ray Imaging. Both of the thesis concerns the development and characterization of X-ray imaging systems based on single photon processing. One can compare measuring the energy of each x-ray to seeing the color of the x-ray, analogous to color imaging in the visible range. “Color” X-ray imaging opens up new perspectives in medical X-ray diagnosis and other applications. The difference in absorption for different colors can be used to discern materials in the object and in principle the elemental composition of an object could be determined and not only the gray scale. For instance, this information might be used to identify contrast media which is used in a large number of examinations in diagnostic x-ray imaging.
Lundqvist and Norlin point out that today's technology make it possible to construct photon counting detector systems that can resolve details to a level of approximately 50 μm. However, there is also a complication with such small pixels since in a semiconductor detector each absorbed X-ray photon creates a cloud of charge which contributes to the image. For high photon energies the size of the charge cloud is comparable to 50 μm and might be distributed between several pixels in the image. Charge sharing is a key problem since, not only is the resolution degenerated, but it also destroys the “color” information in the image. They also outline methods to get around this problem, such as charge summing between adjacent pixels.
A suggestion for a detector for computed tomography for breast imaging is outlined in M. G. Bisogni, A. Del Guerra, N. Lanconelli, A. Lauria, G. Mettivier, M. C. Montesi, D. Panetta, R. Pani, M. G. Quattrocchi, P. Randaccio, V. Rosso and P. Russo “Experimental study of beam hardening artifacts in photon counting breast computed tomography” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Volume 581, Issues 1-2, 21 Oct. 2007, Pages 94-98 This is an example where the energies are so low that Silicon can be used as a detector still maintaining some of the detector efficiency. The X-ray breast Computed Tomography (CT) system is implemented on the gantry of a dedicated single photon emission tomography system for breast Tc-99 imaging. The single photon counting silicon pixel detector was 0.3 mm thick, 256×256 pixel, 55 μm pitch, bump-bonded to the Medipix2 photon counting readout chip. Artifacts may be due to the low detection efficiency and the charge sharing effect of the silicon pixel detector.
Another photon counting detector for low energies is proposed by V. Rosso, N. Belcari, M. G. Bisogni, C. Carpentieri, A. Del Guerra, P. Delogu, G. Mettivier, M. C. Montesi, D. Panetta, M. Quattrocchi, P. Russo and A. Stefanini “Preliminary study of the advantages of X-ray energy selection in CT imaging” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 572, Issue 1, 1 Mar. 2007, Pages 270-273. This detector ensures good detection efficiency (46%) in the used energy range (60 kVp) with a good spatial resolution that arises from a 55 μm square pixel.
Silicon have many advantages as detector material such as high purity and a low energy required for creation of charge carriers (electron-hole pairs) and a also a high mobility for these charge carriers which means it will work even for high rates of x-rays. Not the least it is also readily available in large volumes.
The main challenge with Silicon is its low atomic number and low density which means it has to be made very thick for higher energies to be an efficient absorber. The low atomic number also means the fraction of Compton scattered x-ray photons in the detector will dominate over the Photo absorbed photons which will create problem with the scattered photons since they may induce signals in other pixels in the detector which will be equivalent to noise in those pixels. Silicon has however been used successfully in applications with lower energy as is for example outlined by M. Danielsson, H. Bornefalk, B. Cederström, V. Chmill, B. Hasegawa, M. Lundqvist, D. Nygren and T. Tabár, “Dose-efficient system for digital mammography”, Proc. SPIE, Physics of Medical Imaging, vol. 3977, pp. 239-249 San Diego, 2000. One way to overcome the problem of low absorption efficiency for Silicon is to simply make it very thick, the Silicon is produced in wafers which are approximately 500 μm thick and these wafers can be oriented so that the x-rays are incident edge-on and the depth of silicon may be as much as the diameter of the wafer if required.
Another method to make Silicon deep enough to get high efficiency is advocated in U.S. Pat. No. 5,889,313 Sherwood Parker “Three dimensional architecture for solid state radiation detectors” 1999, this is an inventive method but involves some non standard production methods which may be the reason why it has not been used in commercial imaging detectors.
The first mentioning of crystalline Silicon strip detectors in edge-on geometry as an x-ray detector we could find is R. Nowotny: “Application Of Si-Microstrip-Detectors In Medicine And Structural Analysis” Nuclear Instruments and Methods in Physics Research 226 (1984) 34-39. It concludes that Silicon will work at low energies such as for breast imaging but not for higher energies such as computed tomography mainly because of the higher fraction of Compton scattering and problems related to this.
The edge-on geometry for semiconductor detectors is also suggested in U.S. Pat. No. 4,937,453 Robert Nelson “X-ray detector for radiographic imaging” (edge-on), U.S. Pat. No. 5,434,417 David Nygren “High resolution energy-sensitive digital X-ray” and US2004/0251419 patent application by Robert Nelson. In the US2004/0251419 patent application edge-on detectors are used for so called Compton imaging, in which the energy and direction of the Compton scattered x-ray is measured in order to make an estimation of the energy of the original x-ray. The method of Compton imaging has been much discussed in the literature for a long time but mainly applies to energies higher than what is employed in x-ray imaging, such as Positron Emission Tomography. Compton imaging does not relate to the present invention.
In a paper by S Shoichi Yoshida, Takashi Ohsugi “Application of silicon strip detectors to X-ray computed tomography” Nuclear Instruments and Methods in Physics Research A 541 (2005) 412-420 an implementation of the edge-on concept is outlined. In this implementation thin tungsten plates placed between edge-on silicon strip detector reduces the background of scattered X-rays and improve the image contrast with low dose. The implementation is very similar to what is proposed by R. Nowotny:“Application Of Si-Microstrip-Detectors In Medicine And Structural Analysis” Nuclear Instruments and Methods in Physics Research 226 (1984) 34-39.
Several proposals have been made for photon-counting semiconductor detectors based on high-Z materials such as CdZnTe and clinical images have also been acquired with prototype detectors. The drawback with these materials is the cost and lack of experience in production volumes.
There has been a considerable interest in photon counting detectors in particular for medical imaging but so far there is no working commercial solution at higher energies than around 40 keV. This is because of problems to manufacture detectors in feasible and readily available materials; exotic high Z semiconductors are still expensive and unproven. Silicon has worked for lower energies but for higher energies the problem of high fraction of Compton scatter has been a prohibitive problem together with a working system assembly of a detector that fulfills the geometrical requirements of for example today's CT modalities in terms of combining high detection efficiency in terms of geometry and absorption.